Silicone lubricant containing trialkyl phosphate



Patented July 20, 1954 SILICON E LUBRICANT '5 G ONTAINING .TRIALKYL EHO SPHATE 'ilouglasHfiMoreton, .Pacific Palisades, Calif assignor to Douglas Aircraft Company, -Inc.,

Santa Monica, Calif.

vNo Drawing. ApplicationJune2, .1950, Serial No. 165,868

-Claims. *1 'This'invention relatesito .a silicone oil or liquid having improved properties as aluhrisant.

'Silicone oils or fluids are synthetic relatively high molecular weight liquids, the molecules of which have a skeleton structure of alternating silicon and oxygen atoms bonded oneto another with hydrocarbon groups attached to silicon atoms. Silicone oils or fluids may also be regarded as the liquid silicone polymers orpolymeric organosiloxanes. These silicone oils or fluids areknown and are amply'described in the literature, particularly inithe "following publications: Chemistry of the Silicones, by Eugene G. Rochow, publishedby John Wiley 85.50115, Inc., particularly chapter 1, methyl silicone :oilzis described on pages 54 to 70; Silicone oils partlz Their properties, General Electric Review, November 1946, vol. 49, No. 11, pages 14-18, part II: Their applications, General Electric Review, December 1946, vol. 49, No. 12, pages 28-33, both articles-by Dr Donald F. Wilco'ck;Silicone-lubri- -cants, by T. A. Kauppi and W.'-W. Pederson, Lubrication Engineering, December 1946, vol. 2, No. 4,-page 158,-and February-March 1947, vol.'-3, "No. 1, page 17; :Dimethyl-silicone-polymer fluids and their periormance characteristics in hy- 'draulic systems, by V. Fitzsirnmons, D. L. Pickett, R. O. Militz, and-W A.Zisman,-in' Trans- :actions of the A. S.--M.-E. ior May 1 946,page 3'61: Dimethyl-silicone-polymer fiuids end their performance characteristics 'in unilaterally loaded journal bearings, by J. E. Brophy, -R. O. Militz, and W. A. Zisman, in'Transactions of the A. S. M, E. for-May 1946, page '355, and in references cited therein.

-It is well known that suchsilicone oils or'liquid silicone polymers "have many properties which make them exceedingly desirable'as lubricants. These properties particularly include the rela- :tivel-ysmall change in viscosity with temperature, especially over a wide range-of'temperatura high chemicalstability even at relatively-high temperatures, chemical'inertness and unreactivity,'low pour point, resistance to-shear breakdown, low =-vapor pressure (especially with-volatile fractions removed) high resistance to combustion,-inertness toward available natural and synthetic rubbers, and, where the oilcontains branched chain molecules by introducing trifun ctional-groups into the composition before equilibration, theoils have both very low pour points and high reluctance .to crystallize or solidify, giving them .verydesirable low temperature properties. However, it is ,also known that silicone fluids are lacking in lubricating properties, particularly with respect to extreme pressure and load-carrying. capacity, so that in applications involving high rubbing conditions.

speeds or high unitbearing pressures-the-results are undesirable. In facty'some silicon oils fail as satisfactory lubricants atpressures somewhat lower than-those ordinarily considered to be the initial stages of "the extreme pressure range. Suchlack of lubricity or oiliness of the'silicone fluids, ,particular1y the dimethyl silicone polymer fluids, is especially noticeable wvhere both the loaded surfaces are "ferrous, such assteel' on steel and steel on cast iron.

The situation described in the'preceding paragraph is particularly "true with "regard to the liquid organo-substituted polysiloxanes containing an average of aboutlxQ to 2.5 aliphatic groups per silicon atom as more particularly described in.UnitedStatesPatentgNo; 2,471,850. 1"he-physi cal properties of .such "liquid "organoesubstituted polys'iloxanes makepth-em a 'very desirable liquid for lubrication under hydrodynamic or fiuid film ,M'any ofjthese liquid aliphatic-substituted polysiloxanes have low viscosity .tem-

perature coefiicients, low pour points, and good -not be quite aspronoun'ced where one .of' the rubbing surfaces is a non-ferrous material .such .as

bronze. 'Such liquid aliphatic substitutedpolyslloxanes are compositions comprising essentially silicon atoms connectedto one another by oxygen atoms as illustrated by i the following structure calleda siloxane'structure --Si-O-+Siwherein'a preponderantnumber of the valences of the silicon atoms :are satisfiedby the substitution-thereon"oforganic radicals, e. g., aliphatic radicals. These-compositions of matter'may'be prepared, for example, by the hydrolysis of hydrolyzable aliphatic-substituted silanes, e. -g., dialiphatic-substituted*dihalogenosilanes, for instance, dimethyl dichlorosilane, followed byconiplete or partial condensation of the hydrolysis product. They may also be prepared by hydrolyzing mixtures of "hydrolyzabledialiphaticsubstituted silanes either among themselvesor 'with hydrolyzable silanes containing, vfor example, three aliphatic radicals substituted.onthe silicon atom, for instance, .trimethyl .chlorosilane. More specific 'directionsior the hydrolysis of hydrolyzable aliphatic-substituted silanes to form liquid aliphatic-substituted polysiloxanes may be found, e. g., as indicated in Patent No. 2,471,850, in Patnode applications, Serial Nos. 463,813 (abandoned), 463,8le (Patent No. 2,469,888) and 463,815 (abandoned), filed October 29, 1942, and in Wilcock application, Serial No. 655,162 (pending), filed March 21, 1946. Liquid aliphaticsubstituted polysiloxanes containing an average of about 1.9 to less than 2.0 aliphatic groups per silicon atom may be prepared, for example, by hydrolyzing a mixture of aliphatic-substituted chlorosilanes containing an average of about 1.9 to less than 2.0 aliphatic groups per silicon atom.

By the term hydrolyzable aliphatic-substituted silanes is intended to mean derivatives of SiH4 which contain hydrolyzable groups or radicals, e. g. halogens, amino groups, alkoxy, aryloxy, and acyloxy radicals, etc., in addition to the aliphatic groups substituted directly on the silicon atom that are joined to the silicon through carbon-silicon linkages. Examples of such ali phatic radicals (including alkyl radicals) are, e. g., methyl, ethyl, propyl, butyl, etc.; alicyclic radicals e. g., cyclopentyl, cyclohexyl, etc.; alkenyl radicals, e. g., vinyl, allyl, methallyl, etc. If desired. the above-mentioned radicals may also contain substituents substituted thereon, for instance, halogens.

Hydrolysis of the above silanes or mixtures of t the silanes results in the formation of silanols, i. e., aliphatic-substituted silanes containing hydroxy groups substituted directly on the silicon, which hydroxy groups almost immediately condense intermolecularly (intercondense) splitting out water to give the siloxane linkages mentioned previously. Such intercondensations are accelerated by acidic materials, e. g., sulfuric acid,

hydrochloric acid, ferric chloride, etc., as well as by the basic materials, e. g., sodium hydroxide, ammonium hydroxide, etc. As a result of the hydrolysis and condensation, aliphatic-substituted polysiloxanes may be produced which are partially or completely condensed and which may have on the average up to as high as three organic radicals substituted per silicon atom. The liquid aliphatic-substituted polysiloxanes prepared in this manner consist essentially of silicon atoms joined together by oxygen atoms through silicon-oxygen linkages and aliphatic radicals attached to silicon through carbon-silicon linkages,

the remaining valences, if any, of the silicon atoms being satisfied by hydroxyl radicals and/or 1 EXAMPLE? 1 A composition in accordance with my invention was made up as follows:

90% by volume dimethyl silicone polymer (9981- LT-NV-70) by volume tri(2-ethyl hexyl) phosphate The tri-octyl phosphate was incorporated into the dimethyl silicone polymer by dissolving the tri-octyl phosphate therein. This was accomplished simply by mixing the two components at room temperature. A significant feature of this invention is the discovery that this tri-octyl phosphate can be dissolved in the silicone polymer fluid.

Both the particular silicone polymer and the composition containing the tri-octyl phosphate were tested for extreme pressure properties on a Shell i-ball extreme pressure tester, steel-onsteel. The dimethyl silicone polymer alone was found to have a seizure load at 10 seconds of only 126 kilograms, whereas the composition containing the organic phosphate incorporated therein had a seizure load at 10 seconds of 180 kilograms, a surprising increase of over 40 per cent. It will be observed that the incorporation of the trioctyl phosphate had the effect of raising the seizure load from the unsatisfactory value of 126 kilograms to the satisfactory level of 180 kilograms, thus imparting to the silicone oil desirable extreme pressure properties.

The foregoing improvement in lubricating properties or" the silicone oil also renders it suitable for use as a relatively non-fiammable hydraulic fluid, especially where such lubricating properties are necessary or desirable, and the following tests were made to show the desirable properties for such use:

Autogenous ignition temperature (A. S.

T. M. D286-30) .9 FL. 800 Viscosity at- 210 F centistokes 22.0 F do 58.4 -30 F do 492 The dimethyl silicone polymer of methyl silicone oil used in this Example 1 was obtained from the General Electric Company, Schenectady, New York, under its trade designation 9981LT-NV-70. It is a methylpolysiloxane liquid containing linear methylpolysiloxanes having molecules of finite length. This and similar silicone oils are described in the trade publication of the General Electric Company, Resin and Insulation Materials Division-Chemical Department, Schenectady, N. Y., entitled A Comparative Study of G-E Silicone Oils and Petroleum Oils in Hydraulic Applications and supplemental sheet entitled Available G-E Silicone Oils. The letters LT indicate a silicone oil having extremely low temperature properties. Such a silicone oil is particularly referred to on the third page of the article Silicone oils, part I: Their properties, by Dr. Donald F. Wilcock, General Electric Review, November 1946, in the second paragraph under the heading Physical properties and in the articles by D. F. Wilcock in The Journal of the American Chemical Society, vol. 68, 19 16. The letters NV indicate that volatiles have been removed, particularly the small molecules, by vacuum distillation as a step in their manufacture. The numeral 70 indicates the viscosity in centistokes at 100 F. This dimethyl silicone polymer, 9981LTNV-70, had the following properties:

Autogenous ignition temperature (A. S.

T. M. D286-30) FL- 810 Viscosity at 210 F centistokes 27.8 100 F do 72.7 30 F do 510 The trictyl phosphate was one in which the octyl groups were iso-octyl and specifically 2.-ethyl hexyl. This tri(2-ethyl hexyl) phosphate was found to have the iollowing'propertiesz Autogenous ignition tem- Below 550 F. Too

perature (A. S. T. M. low where non- D286-30) flammability is required.

Viscosity at- 210 F 2.29 centistokes.

100 6.4 centistokes. Viscosity index 90. Viscosity at 40 F 787 centistokes.

Examples 2 to '7 show other compositions made in accordance with my invention using difierent proportions of the dimethyl silicone polymer and tri-octyl phosphate of Example 1. These Examples 2 to 7 are shown along with Example 1, the dimethyl silicone polymer alone and the tri- (Z-ethyl hexyl) phosphate alone in TableI below:

EXAMPLE 1 1 90% by volume dimethyl silicone polymer (9981- LT-NV-) 10% by volume tri 2-ethyl hexyl) phosphate The surprising result was obtained that this tri-octyl phosphate was soluble in the dimethyl silicone polymer at room temperature. The composition had no cloud, and the pour point was below 90* F. The dimethyl silicone polymer used was the same type as that described for Example 1 above except that it was a lighter silicone polymer having a viscosity of only 20 centistokes at 100F.

Table I Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7

Percent Dimethyl Silicone Polymoi 100 90 80 7O 50 20 10 0 Percent Phosphate. 0 10 20 3O 50 70 80 00 100 A. l. T. F. 810 800 740 740' 730 750 750 750 Viscosity (cs 7 219 F A. 27.8 22.0 17.7 14.2 9. 09 5 46 4.09 3.09 2. 29 100 F 72. 7 58. 4 47. 9 39. 7 27 3 l7 2 13. 6 l0. 7 8. 4 -30 F 510 492 496 485 552 515 F 787 Examples 8, 9 and 10 were made from. a di- EXAMPLE l2 methyl silicone polymer from a different source. These example and their properties are shown in Table II below:

The dimethyl silicone polymer oil in these examples was obtained from the Dow Corning Corporation, Midland, Michigan, and this dimethyl silicone polymer and similar polymers are described. in the trade publications of the Dow Corning Corporation, entitled. Dow Corning Fluids and DC Dow Corning Silicones.

Although the dimethyl silicone polymer fluids in the examples above have a viscosity of '70 centistolres at 100 the advantages of my invention may be obtained on any silicone oil, particularly the dimethyl silicone polymer, with different viscosity characteristics. For example, for use as a relatively non-fiammable hydraulic fluid for aircraft, having a pump such as the Vickers pump in the hydraulic system, such a dimethyl silicone polymer having a viscosity of about 20 centistokes at 100 F. will be found more suitable in the compositions of my invention. The advantages of my invention may be obtained for any use to which the silicone oil may be put. Moreover, the advantages of my invention may be obtained regardless of the form in which the silicone oil is used as a lubricant, for example, in accordance with my invention, the suitable 90% dimethyl silicone polymer (DCZ00.-1000) 10% tri(2-ethyl hexyl) phosphate Even in this highly viscous dimethyl silicone polymer, the surprising result of solubility of this tri-octyl phosphate was found at room temperature. This composition was also found to have no cloud, and a pour point of below -90 F.

The dimethyl silicone polymer DC-200-1000 was a member of the Dow Corning Type 200 which is a series of liquid silicones of the homologous series of pure triinethyl end-blocked dimethyl siloxanes ranging in viscosities at 25 C. from 0:65 to 12,500 centistokes. The 0.65- and 2.0- centistoke liquid silicones are essentially pure compounds, hexamethyldisiloxane and dodecamethylpentasiloxane, respectively; all the others are mixtures of silicone polymers. These polymers are more particularly described; in the article entitled Thermal conductivity of liquid silicones, by 0. Kenneth Bates, Industrial and Engineering Chemistry, vol. 41, page 1966,, September 1949. In the designation DC-200-l000, DC-200 indicates the Dow Corning Type 200 liquid silicone polymer, and. the subsequent number, such as 1000 in Example 12 above, indicates the viscosity in eentistokes at 25 C. The DC-20O fluids are also described in the trade publications of the Dow Corning Corporation, entitled Dow Corning Fluids and DC Dow Corning Sili- 0.65 to 1,000,000 centistokes at about 25 C. The properties approach a practically constant value as the polymer size or viscosity approaches 100 centistokes, and for this reason the lower members were originally grouped together and distinguished by the designation DC-500, but this distinguishing designation has been abandoned in the trade and all such fluids are known as DC-200 fluids. -Ience, DC-50070 fluid used in Examples 8, 9 and 10 above is the same as a DC-200 fluid and is now known as DC-200-70 fluid. Also as pointed out in this booklet, the properties of these silicone polymers is due to their chemical structure. The chemical bonds which are most important in such silicone are the silicon-oxygen and silicon-carbon bonds. The simplest silicone may be made from silicon tetrachloride by replacing two chlorine atoms with methyl groups and the resulting product hydrolyzed with water to produce a dimethyl silOxane unit which may be represented by the formula The polydimethyl siloXanes such as the DC-200 fluids are made up of a chain of such units and these polydimethylsiloxanes or DC-200 fluids are made up of chains of varying length as represented by the formula:

OH: H3 0 H! CH;;Si- O-Si OSi-CHI C H3 C H: n C H:

These polydimethylsiloxanes are liquid at room temperature even though 11 may vary from 0 to 2,000 or more. The viscosity of the liquid depends on the average length of the polydimethyl siloxane chain. These polydimethylsiloxanes have chains in which n in the formula above may be from 2 to over 2,000. Viscosities at room temperature of about 25 C. may vary from 0.65 centistoke to 1,000.

EXAMPLE 13 9% by volume dimethy1 silicone polymer (998l- LT-NV-20) by volume tri(normal-butyl) phosphate The tri(normal-butyl) phosphate dissolved in this dimethyl silicone polymer at room temperature. No cloud was found and the pour point was below 90 F.

EXAMPLE 14 90% by volume dimethyl silicone polymer (DC- 200-1000) 10% by volume tri(normal-butyl) phosphate The trii'normal-butyl) phosphate also dissolved in this viscous dimethyl silicone polymer at room temperature. The resulting composition had a cloud point of 10 F. and froze at 80 to 85 F.

The foregoing examples illustrate my invention with respect to a dimethyl silicone polymer or polymethylsiloxane in which the surprising discovery has been made that certain trialkyl phosphates are soluble in the silicone oil and are effective to improve lubricating properties thereof, particularly, to improve the anti-seize property, especially for steel-on-steel. As pointed out above this is quite surprising since most of the additives used for petroleum base lubricants are either insoluble in the silicone oil or ineffective for the purpose added. For example, tri-cresyl phosphate commonly used for petroleum lubrieating oils, will not dissolve in the dimethyl silicone polymers or polymethylsiloxanes.

Although the examples above illustrate the use of tri(normal-butyl) phosphate and a particular tri-octyl phopshate, any of the tri-alkyl phosphat es in which the alkyl groups have from about 3 to 10 carbon atoms may be used. For the heavier polymethylsiloxanes the higher carbon atoms per alkyl group are preferred, as is illus trated by Examples 12 and 14 where a lower pour point is obtained with the tri-octyl phosphate than with the tri-butyl phosphate.

Moreover, although my invention as illustrated in the foregoing example relate particularly to the dimethyl silicone oil, and is particularly suitable for the liquid methyl polysiloxane containing an average of from about 1.9 to 2.5 methyl groups per silicon atom, especially for from about 2.0 to 2.2, it applies also to other silicone oils including other liquid aliphatic-substituted polysiloxanes, for instance, liquid alkyl-substituted polysiloxanes, for example, liquid ethyl-, propyl-, butyl-, isopropyl-substituted polysiloxanes, etc.

Although useful compositions result either with a sufficient proportion of the phosphate in the methyl silicone oil to increase the lubricating properties and with a sufiicient proportion of the silicone oil in the phosphate to improve low temperature properties or fire-resistance of the trioctyl phosphate, or both, the preferred proportions range from sufiicient phosphate to increase the lubricating properties to about 70 volume per cent of the phosphate, particularly as a non-flammable fluid for the hydraulic systems of aircraft.

This application is a continuation-in-part of my co-pending application Serial No. 23,147, filed April 2 1948, now abandoned.

I claim:

1. The liquid lubricating composition consisting essentially of a liquid dimethyl silicone polymer lubricant and a sumcient proportion of trialkyl phosphate in which the alkyl groups have from about 3 to 10 carbon atoms per a lryl group to increase the anti-seize lubricating property of said composition over that of said dimethyl silicone polymer.

2. The liquid lubricating composition consisting essentially of a liquid aliphatic-substituted polysiloxane lubricant containing an average of from about 1.9 to 2.5 aliphatic groups per silicon atom, and a sufiicient proportion of trialkyl phosphate having from 3 to 10 carbon atoms per alkyl group to increase the anti-seize lubricating property of said composition over that of said polysiloxane lubricant.

3. The liquid lubricating composition consisting essentially of a liquid methyl polysiloxane lubricant containing an average of from about 1.9 to 2.5 methyl groups per silicon atom, and trialkyl phosphate having from 3 to 10 carbon atoms per alkyl group to increase the anti-seize lubricating property of said composition over that of said polysiloxane lubricant.

4. The composition as defined in claim 3 in which said tri-alkyl phosphate is tri(2-ethyl hexyl) phosphate.

5. The composition as defined in claim 3 in which said tri-alkyl phosphate is tri(normalbutyl) phosphate.

6. The liquid lubricating composition having satisfactory anti-seize lubricating properties consisting essentially of a liquid dimethyl silicone polymer lubricant deficient in anti-seize properties containing a sufiicient proportion of a trialkyl phosphate with alkyl groups of 3 to 10 carbon atoms dissolved in said liquid dimethyl silicone polymer lubricant to improv the anti-seize {9 properties of said composition over that of said dimethyl silicone polymer lubricant.

7. The composition as defined in claim 6 in which said phosphate is tri(2-ethyl hexyl) phosphate.

8. The composition as defined in claim 6 in which said phosphate is tri(normal-butyl) phosphate.

9. The normally liquid homogeneous lubricating composition consisting essentially of a liquid aliphatic-substituted silicone polymer lubricant having a sufficient proportion of a trialkyl phosphate with alkyl groups of from 3 to 16 carbon atoms dissolved in said silicone polymer to increase the anti-seize lubricating property of said composition over that of said silicone polymer lubricant.

10. The liquid lubricating composition consisting essentially of about percent of liquid dimethyl silicone polymer lubricant having a viscosity of about 70 centistokes at 100 F. and about 19 percent of tri(2-ethyl hexyl) phosphate.

teferences Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,340,331 Knutson Feb. 1, 1944 2,435,124 Bollinger Jan. 2'7, 1948 2,442,741 Morgan et a1. June 1, 1948 2,618,600 Moreton July 15, 1948 2,456,642 Merker Dec. 21, 1948 2,486,493 Revukas Nov. 1, 1949 2,528,535 Merker Nov. '7, 1950 

1. THE LIQUID LUBRICATING COMPOSITION CONSISTING ESSENTIALLY OF A LIQUID DIMETHYL SILICONE POLYMER LUBRICANT AND A SUFFICIENT PROPORTION OF TRIALKYL PHOSPHATE IN WHICH THE ALKYL GROUPS HAVE FROM ABOUT 3 TO 10 CARBON ATOMS PER ALKYL GROUP TO INCREASE THE ANTI-SEIZE LUBRICATING PROPERTY OF SAID COMPOSITION OVER THAT OF SAID DIMETHYL SILICONE POLYMER. 